Lithography exposure apparatuses for the EUV wavelength range (from about 5 nm to about 20 nm) generally comprise a light source, an illumination system for homogeneous illumination of a pattern arranged on a mask with light from the light source, and a projection system for imaging the pattern onto a photo-sensitive substrate. In general, EUV-reflective optical components arranged in such lithography exposure apparatuses are contaminated during use, e.g. by hydrocarbon molecules, the presence of which cannot be avoided even though such apparatuses are operated under vacuum conditions. Moreover, the optical components may be further contaminated by carbon-containing materials due to radiation-induced cracking of hydrocarbon molecules.
Contaminations have an adverse effect on the EUV-reflective optical components, such as mirror elements, as they affect their optical properties, in particular their reflectance, thus leading to increased absorption, and may also introduce wavefront errors. This leads to a short lifetime of these optical elements and a degradation of the overall optical performance of the lithography exposure apparatus.
For taking appropriate measures against contaminations, e.g. replacing optical elements as soon as a certain degree of degradation has been exceeded, the contamination status of the optical components in such lithography systems has to be monitored. For this purpose, a considerable number of measurement methods have been described in the literature. All of these methods are faced with the problem that a contamination measurement should not affect the optical performance of the optical system and, in particular, the vacuum conditions in the system should not be disturbed.
One possibility for performing a contamination measurement is described in detail in EP 1 452 891 A1, which is incorporated herein by reference in its entirety. Therein, a measuring device is described which projects radiation on the optical surface of a mirror component to be inspected inside the projection apparatus, the measuring device subsequently measuring a property of the radiation reflected therefrom in a radiation receiver device. In dependence of at least one property of the reflected radiation, a contamination property of the optical surface is subsequently determined in a processor unit. For this method to work, an additional radiation source and receiver need to be positioned in the optical system to project the radiation to the optical surface and to inspect the radiation reflected therefrom. Moreover, the radiation projected to the optical surface for the measurement may adversely affect the imaging conditions of the exposure apparatus.
In U.S. Pat. No. 6,980,281 B2, incorporated herein by reference in its entirety, a lithographic apparatus is disclosed, in which a beam of radiation passes along a beam path to a substrate. An exchangeable aperture screen is inserted in the beam path to partially block out the beam from a remainder of the path onto the substrate. A sample surface is provided on the aperture screen, so that the sample surface receives a part of the beam that is not passed along the remainder of the beam path. The sample surface is made of a material that is sensitive, under influence of radiation from the beam, to chemical alterations that also affect the optical element under influence of radiation from the beam. The sample surface is later analysed for chemical alterations after exposure to the beam. However, additional straylight may be generated during the measurement process by the radiation reflected from the sample surface, as, even though reflected away from the beam path, some of the radiation may return to the beam path through scattering at components present in the vacuum vessel outside of the beam path. Also, for a chemical analysis of the sample surface, the aperture screen has to be removed from the lithographic system. Consequently, no inspection of the sample surface is possible during the exposure process.